As digital transmission rates increase to ever higher levels, the effects of distortion of all kinds become more serious. In digital radio transmission systems, for example, where quadrature amplitude modulation (QAM) is common, higher bit rates are accommodated by not only higher baud rates but also more analog levels for each symbol. It is obviously more difficult to accurately differentiate one level out of sixteen in a 256 QAM system than one level out of 4 in a 16 QAM system.
When the source of the distortion is linear, there are many known methods of providing correction, among the most effective being the digital transversal equalizer. When the distortion is nonlinear, however, the known digital equalizers being linear, provide inadequate improvement. Because the power amplifiers of radio systems generate substantial nonlinear distortion, special nonlinear analog circuits are often used to create a similar distortion which is then subtracted from the signal. Such analog circuits, however, require precise adjustment initially and readjustment with aging.
Attempts to provide nonlinear distortion correction with digital circuitry have to date been disappointing. Previous digital circuits fall into two broad categories, those that take into account the spreading of the signals over time as a result of limited transmission channel bandwidth, and those that do not. The circuits that do not, have limited performance as a result. The nonlinearity correction circuits that do account for time spreading have generally been too complex to implement in a practical way. For example, in "Cancellation of Power Amplifier Nonlinearities in Digital Radio Receivers", Proceedings, ICC-87, Volume 3, pages 1809-1814, H. Sari and G. Karam have described such a linearizer that takes into account only three symbols, yet needs 21 taps, 18 of which are nonlinear. To obtain the accuracy of a five symbol span, 35 taps would be required.
An object of this invention is a nonlinear digital linearizer that takes signal spreading into account, yet is simple and practical. Furthermore, it can readily be made adaptive to eliminate any need for manual adjustment.